This invention relates to a coating for use on mirror backs, and, in particular, to a lead-free organic coating to be applied to the metallic film layer on the back of a mirror to protect the metallic layer and prevent corrosion thereof.
Typical mirrors are made of a glass sheet and a thin layer of a metallic film applied to the back of the sheet. The metallic film layer adhered directly to the glass is usually a film of silver, although other metallic films may also be used, such as copper. When silver is used as the primary reflective layer, it is commonly protected by a second metallic film layer of copper or some other metal.
It has long been known to employ various paints and other film-forming organic resins as a further protective layer over a metallic film layer to protect the layer from corrosion and physical damage. Traditionally, these paints have included lead-based corrosion inhibitors, such as lead salts. However, in recent times, both users and producers of such paint products have sought to eliminate the use of lead and lead compounds for health and environmental reasons. A recent effort in this direction is reflected in U.S. Pat. No. 4,707,405 to Evans et al directed to use of cyanamide salts of non-lead metals as corrosion inhibitive pigments in mirror back coatings. This patent discloses the use of such non-lead cyanamide salts as calcium cyanamide and zinc cyanamide in various types of film-forming thermosetting or thermoplastic resins which are applied over the silver and copper layers on the backs of mirrors. Continuing efforts have been made to produce an effective lead-free corrosion inhibiting mirror back coating to satisfy the long felt need of the industry, but to date, no such commercial coatings have proven to be as effective in inhibiting corrosion and overall protection as lead containing coatings.
Bearing in mind the problems and deficiencies of the prior art, and the long felt need of industry, it is therefore an object of the present invention to provide an effective corrosion inhibiting, lead-free coating for covering the metallic film layers on the back of a mirror.
It is another object of the present invention to provide an organic film-forming resin which incorporates a non-lead corrosion inhibitor which can be easily applied by existing techniques to mirror backs.
It is a further object of the present invention to provide a lead-free paint for protecting thin silver and/or copper or other metallic film layers which imparts corrosion protection of equal to or greater effectiveness than prior art lead containing paints.
It is yet another object of the present invention to provide an effective process for inhibiting corrosion of metallic film layers on mirrors.
It is a further object of the present invention to provide mirror and other articles having effective protection of their metallic film layers against salt spray and other corrosion-causing compounds.